1. Field of the Invention
The present invention relates to a circuit and a method for compensating for a nonlinear distortion of a power amplifier, and more particularly to a transmitting circuit having a nonlinear distortion compensating circuit.
2. Description of the Related Art
Digital microwave radio communication systems generally employ a quadrature amplitude modulation format such as a multilevel quadrature amplitude modulation format from the standpoint of frequency utilization efficiency. According to the quadrature amplitude modulation format, since a power amplifier for amplifying a transmission signal uses only a linear region of its input vs. output characteristics, it is desirable to have a sufficiently large backoff (which represents an operating point and is generally given as the difference between a maximum output amplitude level and a saturated output power level). If the backoff is large, however, a sufficiently large amount of transmission power cannot be obtained. Actually, therefore, it is necessary to reduce the backoff for the power amplifier to use a nonlinear region of its input vs. output characteristics. As a result, a problem arises in that a nonlinear distortion caused when the power amplifier uses the nonlinear region is added to a transmission signal.
To solve the above problem, there has been devised a process of compensating for a nonlinear distortion produced when a transmission signal is amplified by adding an inversion of the nonlinear distortion depending on the power of an input signal to the transmission signal using a circuit referred to as a predistorter. Heretofore, such predistorters comprise an analog circuit for use in the RF band. However, the conventional predistorters suffer limitations on their compensation accuracy on account of their component variations, and are difficult to make adjustments. In recent years, the advance of digital signal processing technologies has led to predistorters that are constructed as baseband digital circuits.
Transmission circuits using digital predistorters are generally classified into two types, i.e., open-loop transmission circuits and closed-loop transmission circuits. FIG. 1 of the accompanying drawings shows a typical open-loop transmission circuit arrangement (see patent document 1 (Japanese laid-open patent publication No. 2001-53627)), and FIG. 2 of the accompanying drawings shows a typical closed-loop transmission circuit arrangement (see patent document 2 (Japanese laid-open patent publication No. 2000-228643)).
The open-loop transmission circuit shown in FIG. 1 comprises FIR filter 10, predistorter 11, modulator 12, and power amplifier 13 which are connected in series. An input baseband digital signal (Ich DATA, Qch DATA) is supplied through FIR filter 10 and predistorter 11 to modulator 12, which performs quadrature amplitude modulation on the signal. The modulated signal is then amplified by power amplifier 13. Inverse characteristics of the nonlinear distortion of power amplifier 13 are determined in advance and held in predistorter 11 to determine a compensation value with respect to the power level of the input signal. The circuit arrangement shown in FIG. 1 is advantageous in that it is simple and inexpensive. However, since the inverse characteristics held in predistorter 11 are of fixed nature, if the inverse characteristics held in predistorter 11 and the actual inverse characteristics are different from each other for some reason, then the open-loop transmission circuit fails to provide a sufficient nonlinear distortion compensating capability.
The closed-loop transmission circuit shown in FIG. 2 differs from the open-loop transmission circuit shown in FIG. 1 in that it has adaptive predistorter 14 in place of predistorter 11 shown in FIG. 1, and additionally has comparison/control circuit 15 and demodulator 16. In operation, the modulated signal that is amplified by power amplifier 13 is demodulated by demodulator 16. Comparison/control circuit 15 compares the baseband digital signal (Ich DATA, Qch DATA) output from FIR filter 10 and the demodulated signal from demodulator 16 with each other, and adaptively changes the amount of compensation in adaptive predistorter 14 in order to equalize the baseband digital signal and the demodulated signal to each other. Adaptive predistorter 14 can thus compensate for the nonlinear distortion optimally at all times.
It is generally known that the input vs. output characteristics of the power amplifier vary with the operating temperature thereof. With the open-loop arrangement, since the amount of compensation is determined based on only the power of the input signal, as described above, if the temperature of the power amplifier changes, then the amount of compensation and the actual inverse characteristics become different from each other, resulting in an in sufficient nonlinear distortion compensating capability. The closed-loop arrangement is free of the above drawback because characteristic changes due to changes in the temperature of the power amplifier are adaptively compensated for. However, the closed-loop transmission circuit is much more complex in circuit arrangement and costly than the open-loop transmission circuit because the closed-loop transmission circuit requires the demodulator. Accordingly, it is desirable from the standpoint of simpler and less costly circuit arrangements to be able to achieve temperature compensation in the open-loop transmission circuit.
According to one conventional process of achieving temperature compensation in the open-loop transmission circuit, a plurality of compensatory values corresponding to a plurality of temperatures are stored in a table, and a predistorter acquires a compensatory value depending on the operating temperature of a power amplifier from the table (see patent document 3 (Japanese laid-open patent publication No. 2001-274851)).
With the arrangement disclosed in patent document 3 referred to above, it is necessary to store temperatures at small intervals in the table for providing an exact match between a compensatory value retrieved from the table and the amount of a nonlinear distortion produced in the power amplifier at the actual operating temperature. However, storing temperatures at small intervals in the table requires that a memory which holds the table be larger in circuit scale and hence more expensive. For this reason, the arrangement disclosed in patent document 3 has to store temperatures at certain compromising intervals in the table. Consequently, it is difficult even for the arrangement disclosed in patent document 3 to achieve an exact match between a retrieved compensatory value and the amount of a nonlinear distortion produced at the actual operating temperature, and to provide a sufficient temperature compensating capability.